# Recent content by ylem

1. ### What do we mean when we say on shell particles?

Thank you all very much for your help. :-) However, how could I tell if a process is possible for on-shell particles? So... picking a simple process a photon annihilating to give an electron and positron. How can I determine if this is possible for on-shell particles? Cheers, Samantha
2. ### Scattering matrix to traces/dirac spinors

This is right. The reason it's a trace is because the definition of the trace is the sum of all the diagonal elements, Tr A = Sum A_{ii} (Sorry, I don't know how to get Latex stuff on here!). So, you have the same index at the start and the end of your T-matrix element, that's why it's a trace...
3. ### What do we mean when we say on shell particles?

What do we mean when we say "on shell" particles? Hello! What do we mean when we say a particle in on shell? What is off shell? I gather it is related to energy and momentum conservation to say if a process is possible, but I've no idea what it actually is. Is it something to do with...
4. ### Pion Decay to Muons and not Electrons

Hello! I'm afraid I've asked this before, a while ago, but I still don't completely understand. It's not a homework question as such, but I thought this might be an appropriate place to put it. Homework Statement Why is it that pions decay via pi^+ --> mu^+ + antimuon neutrino 98% of...
5. ### Units in Particle Physics

Hello! I was wondering if someone could help me with a problem of units! I'm currently deriving probability equations for neutrino oscillations and I'm stuck with units! I have: [(mass)^2 * time]/[4*energy] and I want to write it as [1.27 * (mass)^2 * distance]/[Energy] with distance in...
6. ### Negative Pion Decay

Hello! I was just wondering something... Why is it that a negative pion always decays into a muon and antimuon neutrino? Why not an electron and antielectron neutrino? (and the same for a positive pion) Any answers would be grately appreciated :-) Samantha
7. ### Injective and surjective?

Yeah! Thanks a lot :-)
8. ### Injective and surjective?

Hello! I hope I've posted this in the correct section... I'm a 3rd year undergraduate and we're currently studying Vector Spaces (in QM) and I just don't understand what injective (one-to-one) and surjective (onto) mean? As a result I have no idea what an isomorphism is! I realise this is...
9. ### Probability of Neutrino Transformations

Homework Statement I have a questions on Neutrino Oscillations, but i have no idea how to solve any of it... this is just one part... How do I show that the probability of a mu neutrino having transformed into a tau neutrino at a time t is: sin^2(2theta)sin^2[((difference in masses...
10. ### Neutrino Oscillations

Thanks! That's really helpful! :-) x
11. ### (Kinetic Energy) Operator as a Matrix

Thanks so much for your help! I think I understand it a bit more now! :-) x
12. ### (Kinetic Energy) Operator as a Matrix

Hey! I am completely confused... In quantum mechanics at University we are learning about operators. I don't understand what you actually have to do to write an operator as a matrix... In my notes, I'm looking at the kinetic energy operator in the Hamiltonian... I can see for something...
13. ### Is 99% of the volume of the universe empty?

Is it not something to do with the fact that light does not have a wavelength small enough for us to see the spaces, as they're so small... so we have to use gamma rays ones and what not? I really have no clue... just guessing!
14. ### Neutrino Oscillations

I have a few questions about Neutrino Oscillations... Firstly, why are they called oscillations? Is it because they can go back and forth to different flavours/masses? Next, if, for example, a tau neutrino is produced in the sun, will it oscillate to the other flavours on its way to earth...
15. ### Electric Field Due to Point Charges?

Thanks! This is where I'm getting confused though, because if the y components cancel then when I find the magnitude of the field I will just be squaring the x components, so I won't have anything to the power of a half to expand using the Binomial? :confused: